Microfluidic chips can simplify a wide range of analytics because they are compact, inexpensive and easily fabricated. However, chips used for fluorescence analysis require a light source. Most light sources are larger than the volume they must illuminate, which makes the entire package larger. Researchers at Cornell University in Ithaca, N.Y., have developed a method for creating light-emitting nanofibers that could put a light source directly into a microfluidic chip. The nanofibers are based on ionic transition metal complexes and are produced by electrospinning, which makes them easy to fabricate without expensive technology.

According to José M. Moran-Mirabal, lead author of a recent paper on the technique, electrospinning is like pouring syrup onto a pancake on a rotating table while the syrup jet is maintained at a fixed position. A polymer solution containing a ruthenium compound in dry acetonitrile is applied to a silicon substrate with micropatterned gold interdigitated electrodes. This is done by applying a high voltage between a tip and substrate. As the solution leaves the tip, it forms a jet that is stretched and thinned; the acetonitrile evaporates in flight, depositing a solid fiber on the substrate. The viscosity of the solution determines the thickness of the deposited fiber.

Electrospinning allows researchers to deposit a fine polymer fiber onto a substrate. A high voltage applied to a sharp tip creates a jet of polymer solution containing transition metal complexes (red). The jet accelerates toward the substrate (blue) containing micropatterned electrodes (gold). The jet solidifies in flight and is deposited as a continuous solid fiber. Applying a voltage to the electrodes creates light emission from a thin region where the fiber crosses the two oppositely polarized electrodes. Images courtesy of José Moran-Mirabal.The fibers are deposited on the substrate such that a voltage can be applied between pairs of electrodes to produce light in a given segment of the fiber, Moran-Mirabal explained. “The current flows from one electrode to the next through the fiber segment spanning them.” He and his colleagues tested substrates with electrodes spaced at 500 nm or at 5 μm. A small-diameter fiber on 500-nm interdigitated electrodes produced a light-emitting spot of 240 × 325 nm, small enough to be limited by the optical diffraction limit.

Each substrate has multiple locations where the fiber emits the 600-nm (red) light. The group tested the fibers by ramping the voltages up and down and by operating them continuously for up to 10 hours at 100 V. For substrates with the 5-μm electrode spacing, 10 V was enough to create emission. The smaller ones with 500-nm electrode spacing needed only 2.6 V.

Researchers hope to integrate light sources into microfluidic chips using an electrospun fiber (green) that emits 600-nm light (red) where it crosses the gap between two electrodes on a substrate (white lines).Moran-Mirabal said that the group has not yet integrated the fibers onto an actual microfluidic chip because the polymer in the fibers is water soluble and thus cannot be in direct contact with the solution containing the molecules. One potential solution, he explained, is to mix the light-emitting elements in other polymers, such as poly(methylmethacrylate), which also can be electrospun.

The group plans to test light emission from fibers made from metal complexes dissolved in polymers that are compatible with aqueous solutions, he added. That includes studying different metal complexes that emit at other wavelengths. He said that they will later tackle the problem of integrating the fibers onto a chip.